Safety current supply for photoelectric light sources



March 8, 1949. CRANCH 2,464,074

SAFETY CURRENT SUPPLY FOR PHOTOELECTRIC LIGHT SOURCES Filed April 8, 1947 IN VEN TOR.

Patented Mar. 8, 194-9 UNITED STATES PATENT OFFICE SAFETY CURRENT SUPPLY FQR. PHOTO- ELE'CTHIC LIGHT SOURCES Eliot A. Cranch, Denver, Colo. Application April 8, 1947, Serial No. 340,238

1 Claim. 1

This invention elates to a safety photo-elem trio cell circuit. Photo-electric cells are used for starting or operating machines of various kinds in response to the breaking of a light beam by a work piece being placed in the machine. In the usual photo-electric circuit for this use a relay or relays are employed for closin a main circuit. The relay is energized from a control circuit in which the current flow increases as the light supply to a photo-electric cell decreases. The lamp sup ying the light beam to the photoelectric cell energized independently of the control circuit. Therefore, should the current supply to the lamp fail, or should the lamp itself burn out, the control circuit will be energized to operate the relay, closing the main circuit. This, of course, is very dangerous, for it will start the machine beingcontrolled at unpredictable times, and keep it in operation until manually stopped by the attendant.

The principal object of this invention is to provide a circuit in which the lamp will be placed in series with the control circuit so that, should the light source fail for any reason, the control circuit will be tie-energized to open the main circuit relay and prevent starting of the machine until repairs or replacements have been made.

Other objects and advantages reside in the detail construction of the invention, which is designed for simplicity, economy, and efficiency. These will become more apparent from the following description.

In the following detailed description of the invention, reference is had to the accompanying drawing which forms a part hereof. Like numerals refer to like parts in all views of the drawing and throughout the description.

The invention involves but little change in the conventional circuit and is best described and understood by reference and comparison with the conventional standard circuit. For this purpose, such a conventional circuit is illustrated in 1 of the drawing, and the same circuit changed to adapt it to the objects of this invention is illustrated in Fig. 2.

The conventional elements of a photo-electric control circuit are indicated diagrammatically on the drawing and designated by numeral as follows: main circuit it, to be controlled by the photo-electric circuit; relay H, for closing main circuit it; photo-electric cell it; lamp l3, for illuminating a photo-electric cell; reflector I4; condensing lenses l5; light beam Iii; electrolytic tube ll; balancing resistors l8 and I9; grid leak The circuit of Fig. 1 is conventional. The current for operating the lamp it is supplied from an outside source through an independent lamp circuit The control circuit is supplied With half-wave rectified current through the rectifier tube which is connected through the medium of a rectn r conductor 2? and a relay conductor 33% with the relay ll. The tube 2! is supplied from one of the alternating current leads 22. The other lead feeds the grid of the tube 1'! t rough a conductor the resistor l8, and a c -ode conductor The plate current from the tube ll is conducted to the relay it through a plate conductor 26. The circuit is bridged by a shunt conductor in series with the balance resistor l 9. The resistor ii] is of a value which will impress a proper bias on the cathode of the tube ll.

The flow of the bias current is controlled by the grid of the tube ll, which is energized from the photo-electric cell 52 through a grid conductor 29. The grid 29 and the cathode 28 are of the same polarity so that when the photo-electric cell i2 is energizing the grid of the tube ll, no current will flow through the relay ll. Should the light beam I 5, however, be interrupted, no current will flow to the grid, so that the full bias of the tube ll will flow to the plate conductor 26, causing the relay H to close the main circuit l0.

As thus far described, and as illustrated in Fig. l, the circuit is the conventional one. It was thought best to first describe and illustrate the conventional circuit in order that the changes and additions thereto would be more readily understandable.

It can be readily seen that should the lamp circuit 323 fail, or should the lamp l3 burn out, the relay ll will be energized, closing the main circuit 50. This closure may occur at a time to do great damage if the circuit is being used for machine control, or if used for pump control the pump will operate until stopped manually.

This invention is designed to cause the relay H to remain open should any failure occur in the light source. This is accomplished by very slight changes in the conventional circuit of Fig. 1.

The outside lamp circuit 23 and the relay conductor 3! are eliminated and the lamp [3 is placed in series, by means of a lamp conductor 30 and a lamp-relay conductor 34, between the relay H and the rectifying tube 2|, so that all current flowing through the relay ll must also flow through the lamp iii. The circuit for this relay current may be traced as follows: from one of the supply leads 22 through the conductor 24, the resistor i8, the cathode conductor 28, the plate conductor 26, the relay ll, thence through the conductor 34, the lamp l3, and the rectifier conductor 2?, thence back to the other supply lead 22. This circuit and this current serve no function in illuminating the lamp l3.

The current for activating the filament of the lamp I3 is obtained by bridging a condenser circuit 32 between one of the supply leads 22 and the lamp-relay conductor 36. The latter circuit is broken by a condenser 33 which allows the alternating current from the leads 22 to pass, but prevents the half-wave current from the relay H from being shorted across the lamp It.

The lamp energizing circuit may be traced as follows: from one of the alternating current supply leads 22 through the conductors 2 1 and 25 and resistors l8 and it to the conductor 30, thence through the lamp it, the lamp-relay conductor 34, the bridging conductor 32 and the condenser 33 to the opposite supply lead 22.

It can be seen from the above that the photoelectric control circuit will operate similarly to the conventional one of Fig. 1. It will also be seen that the lamp it will remain illuminated as long as current is supplied through the leads 22. Should the lamp l3 burn out, however, the half-wave relay circuit is broken at this point, for current cannot flow from the relay H to the rectifier tube 2i. Therefore, the relay cannot operate and the main circuit 60 will remain open until repairs or replacements are made.

While a specific form of the improvement has been described and illustrated herein, it is desired to be understood that the same may be varied, within the scope of the appended claim, without departing from the spirit of the invention.

Having thus described the invention, what is claimed and desired secured by Letters Patent is:

A photo-electric cell control circuit comprising: a rectifier; a gridmontrolled electronic tube; a photo-electric cell supplying current to the grid of said tube; a lamp supplying a light beam to said photo-electric cell; a relay; a pair of alternating current supply leads, one of said leads being connected to the cathodes of said photo-electric tube and said electronic tube, the second supply lead being connected successively in series through said rectifier, said lamp, and said relay to the plate of said tube; and a bridging conductor extending from the second supply lead to a connection between said lamp and said relay to supply alternating current to energize said lamp, said bridging conductor including a condenser to prevent short-circuiting of the rectified current flowing through said lamp.

ELIOT A. CRANCH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,060,500 Hitchcock Nov. 10, 1936 2269.340 Gulliksen Jan. 6, 1942 

